Interestingly, the different response to SERM therapy between these two groups of cancers was significant only in patients that were also negative for the epidermal growth factor 2 (HER2-), but not in HER2+ tumors, suggesting that the PR positively affects the response of breast cancers to endocrine therapy, especially when the HER2 signaling is inactivated [24]. Here, ERBB2 is linked to breast cancer.